This invention relates to an injection molding apparatus for the injection molding of data storage discs of thermoplastic synthetic material having information pits on one side, particularly compact discs of polycarbonate or polymethylmethacrylate. The apparatus includes a movable die having an embossing plate mounted thereon confronting the fixed die of the apparatus for embossing the information pits on the one side of the discs during the molding operation. And, the fixed die has an insert mounted thereon confronting the embossing plate and forming a die next therewith in a closed position of the dies. The insert is in the form of a plate having a die opening through which the thermoplastic synthetic material is injected into the die nest formed between the dies.
The production of data storage discs of thermoplastic material, such as compact discs, video discs, CD-Rom discs, Draw and EDRAW discs is carried out by injection molding machines during a molding operation in which considerable demands are placed on the production process. For example, the work areas in which these data storage discs are produced must normally be of oustanding cleanliness and, after the injection molding operation, the discs must be removed by robots from the injection molding dies.
The injection molding process for producing such data storage discs is much more difficult than the procedure for the production of more common injection molded parts. For example, it is necessary that the compact discs which are later to be scanned by laser beams be absolutely plane and smooth on the side from which the scanning by the laser beam takes place, and it is necessary that the information pits, having dimensions n the range of about 0.2 micron, be produced on the reverse side during molding with utmost precision. Moreover, it is necessary that during the injection molding process the injected material be cooled sufficiently slowly so that a desired structure and material orientation is obtained particularly to avoid high orientation levels. Considerable difficulties also result if the thermoplastic injected material is processed just below its decomposition temperature. This requires that even the median time that the plastic material remains in the plastification and injection nozzle be as short as possible, so that no thermal decomposition appears.
Apart from these rigid demands during the injection molding process, there are also special requirements for the injection molding dies which typically comprise a die half mounted on a movable die plate and a die half mounted on a fixed die plate. The movable die has an embossing plate for embossing the information pits on one side of the discs during the molding operation, while the fixed die has a plane surface confronting the embossing die. And, the fixed die typically has a die opening through which the thermoplastic synthetic material is injected into the die nest formed between the dies.
The movable die may be in the form of a male conical part received by a complementarily formed female fixed die during the closing operation, such that the dies form a die nest when closed for the production of the compact disc. In such closed position, the plastified synthetic material, preferably a plastified polycarbonate, is injected through the die opening of the fixed die and into the die nest formed between the dies.
Other than the fact that the female fixed die must have a plane and completely smooth surface confronting the embossing plate on the movable die, during sequential injection cycles care must be taken to avoid accumulation in this plane area of residual or extraneous synthetic material which could take place if the processing temperature elevated excessively, or if the time that the plastified synthetic material remains in the platification and injection nozzle is too long, such that the thermal decomposition of the material will occur.
Moreover, care must be taken that the flat area of the fixed die confronting the embossing plate is alway brightly polished and essentially non-porous, such that the diameter of the surface pores cannot be detected when subsequently scanning the information pits. Generally, the pore size should be as close to actual zero as possible, i.e., a practically pore-free polished surface should be provided in this area. Before the injection molding machine is operated, these polished areas must be washed clean at 50.degree. C. with a non-corrosive cleaning solution, since even slight dirt remnants will render the cast compact discs unusable, since a perfect optical scanning could not be carried out.
Other interfering influences can take place during the injection molding operation, such that after the operation, moisture can precipitate in the polished flat area of the fixed die, the adhering mineral components of which could also result in rejection of the compact dies after molding. This is generally also applicable for other data storage discs which are laser scanned. A plane surface is also required when the scanning is carried out electromagnetically, such as with video discs, with the scanning heads at a distance from the scanning surface that measures in the range of several microns.
The injection molding dies for the injection of data storage discs generally have cooling and heating channels for known purposes.
The aforementioned problems persist since the mold dies must be of metal or metal alloys required for the injection molding machine.